I wish there were databooks in this franchise. Then we might be able to know for sure whether or not Butterfree was supposed to evolve from Venonat. At the moment I think it's possible that they could've been planned to be related to each other at some point. I originally didn't think that it could be the case but I've been reading stuff about beta designs recently and I think it's possible that those two were planned to be part of the same evolutionary family.